Beyond an eminent role in hemostasis and thrombosis, platelets are characterized by expert functions in assisting and modulating inflammatory reactions and immune responses. This is achieved by the regulated expression of adhesive and immune receptors on the platelet surface and by the release of a multitude of secretory products including inflammatory mediators and cytokines, which can mediate the interaction with leukocytes and enhance their recruitment. In addition, platelets are characterized by an enormous surface area and open canalicular system, which in concert with specialized recognition receptors may contribute to the engulfment of serum components, antigens, and pathogens. Platelet-dependent increases in leukocyte adhesion may not only account for an exacerbation of atherosclerosis, for arterial repair processes, but also for lymphocyte trafficking during adaptive immunity and host defense. This review compiles a selection of platelet-derived tools for bridging inflammation and vascular disease and highlights the molecular key components governing platelet-mediated mechanisms operative in immune surveillance, vascular remodeling, and atherosclerosis.

Background—Circulating platelets and chemoattractant proteins, such as the CC chemokine RANTES, contribute to the activation and interaction of monocytes and endothelium and may thereby play a pivotal role in the pathogenesis of inflammatory and atherosclerotic disease.Methods and Results—The binding of RANTES to human endothelial cells was detected by ELISA or immunofluorescence after perfusion with platelets or exposure to their supernatants. Monocyte arrest on endothelial monolayers or surface-adherent platelets was studied with a parallel-wall flow chamber and video microscopy. We show that RANTES secreted by thrombin-stimulated platelets is immobilized on the surface of inflamed microvascular or aortic endothelium and triggers shear-resistant monocyte arrest under flow conditions, as shown by inhibition with the RANTES receptor antagonist Met-RANTES or a blocking RANTES antibody. Deposition of RANTES and its effects requires endothelial activation, eg, by interleukin-1β, and is not supported by venous endothelium or adherent platelets. Immunohistochemistry revealed that RANTES is present on the luminal surface of carotid arteries of apolipoprotein E–deficient mice with early atherosclerotic lesions after wire-induced injury or cytokine exposure. In a mechanistic model of atherogenesis, monocyte adherence on endothelium covering such lesions was studied in murine carotid arteries perfused ex vivo, showing that the accumulation of monocytic cells in these carotid arteries involved RANTES receptors.Conclusions—The deposition of RANTES by platelets triggers shear-resistant monocyte arrest on inflamed or atherosclerotic endothelium. Delivery of RANTES by platelets may epitomize a novel principle relevant to inflammatory or atherogenic monocyte recruitment from the circulation.

The subset of human blood monocytes expressing low levels of CD14 and high levels of CD16 (CD14⁺CD16⁺) exhibits features resembling mature tissue macrophages and can be expanded in inflammatory conditions. We analyzed expression of CC chemokine receptors (CCR) in CD14⁺CD16⁺ versus CD14⁺⁺ monocytes, which may be crucial for specific trafficking. Multicolor flow cytometric analysis of whole peripheral blood revealed that, as opposed to CD14⁺⁺ monocytes, the CD14⁺CD16⁺ subset lacked surface expression of monocyte chemotactic protein-1 (MCP-1) receptor CCR2, however, it showed significantly higher surface expression of the macrophage inflammatory protein 1α (MIP-1α)/RANTES receptor CCR5. This was paralleled by differences in mRNA expression in the subsets, as shown by reverse transcriptase-polymerase chain reaction using sorted cells. In comparison to CD14⁺⁺ monocytes, CD14⁺CD16⁺ cells expressed lower CCR2 but higher CCR5 transcript levels, whereas CCR1 levels were equivalent. Flow cytometric analysis of isolated human monocytes recovered after transendothelial chemotaxis assays revealed that the percentage of CD14⁺CD16⁺ cells was dramatically reduced in the fraction migrating toward MCP-1 compared with the fraction that did not migrate or the input, showing that polarized CCR2 expression was accompanied by a differential chemotactic responsiveness. Moreover, CD11b surface expression was preferentially up-regulated by MCP-1 in CD14⁺⁺ cells but by MIP-1α in CD14⁺CD16⁺ monocytes, confirming the functional relevance of distinct CCR expression. The characteristics of CD14⁺CD16⁺ cells may reflect preactivation by cytokines and determine their predilective localization during specific inflammatory conditions or susceptibility to infection. J. Leukoc. Biol. 67: 699–704; 2000.

Objective— Platelet activation mediates multiple cellular responses, including secretion of chemokines such as RANTES (CCL5), and formation of platelet microparticles (PMPs). We studied the role of PMPs in delivering RANTES and promoting monocyte recruitment. Methods and Results— Here we show that PMPs contain substantial amounts of RANTES and deposit RANTES on activated endothelium or murine atherosclerotic carotid arteries. RANTES deposition is facilitated by flow conditions and more efficient than that conferred by PMP supernatants. Interactions of PMPs with activated endothelium in flow were mostly characterized by rolling. RANTES deposition showed a diffuse distribution pattern and was rarely colocalized with firmly adherent PMPs, substantiating that RANTES deposition occurs during transient interactions. Importantly, preperfusion with PMPs enhanced monocyte arrest on activated endothelium or atherosclerotic carotid arteries, which could be inhibited by a blocking antibody or a RANTES receptor antagonist. Blockade or deficiency of PMP-expressed adhesion receptors demonstrated differential requirement of P-selectin, glycoprotein Ib (GPIb), GPIIb/IIIa, and junctional adhesion molecule-A for PMP interactions with endothelium, PMP-dependent RANTES deposition, and subsequent monocyte arrest. Conclusion— Circulating PMPs may serve as a finely tuned transcellular delivery system for RANTES, triggering monocyte arrest to inflamed and atherosclerotic endothelium, introducing a novel mechanism for platelet-dependent monocyte recruitment in inflammation and atherosclerosis.

The chemokines platelet factor 4 (PF4) and RANTES (regulated on activation normal T cell expressed and secreted) are secreted by activated platelets and influence multiple cell types and biologic processes. For instance, PF4 inhibits progenitor cell proliferation and angiogenesis, while platelet-derived RANTES is involved in vascular recruitment of monocytes. However, little is known about functional interactions of PF4 and RANTES. Here we show that the presence of PF4 enhanced the arrest of RANTES-stimulated monocytes and monocytic cells on activated endothelial cells under flow conditions, while binding of PF4 to the monocyte surface was increased by RANTES. Both RANTES-triggered arrest and PF4 binding involved monocytic chondroitin sulfate. Ligand blots and surface plasmon resonance revealed a robust heterophilic interaction of PF4 with RANTES but not with RANTES variants defective in higher order oligomerization. The tetrameric mutant E26A bound to the monocyte surface without increasing PF4 binding, and monocyte arrest induced by E26A-RANTES was not enhanced by PF4. Stimulation of monocytes with supernatants of activated platelets triggered arrest involving RANTES and PF4, as shown by inhibition studies. Our results suggest that heterophilic interactions with PF4 require structural motifs important in RANTES oligomerization and amplify RANTES-triggered effects on monocyte adhesion. This may have implications for the modulation of inflammatory recruitment by platelet-derived chemokines.